Rootstocks constitute a very important resource of grafted plants, especially in terms of improving the production of watermelon. In this work, a new rootstock, i.e. ‘Wanzhen No. 2’, was used to evaluate the grafting effect on three watermelon cultivars (‘Xiuli’, ‘Jinlvhongling’, and ‘Xinong No. 8’) subjected to double-root-cutting grafting. Protected field trials were also conducted. The obtained results have shown that the double-root-cutting grafting technique significantly increases the root length and dry weight of ‘Wanzhen No. 2’, when compared to bottle gourd rootstock. ‘Wanzhen No. 2’ was also found to significantly increase the average survival rate of grafted seedlings, up to 96% in three experimental bases. The fruits harvested from the ‘Wanzhen No. 2’ grafted plants had higher weight than those obtained from non-grafted plants, without affecting the central sugar content. By using ‘Wanzhen No. 2’ as the rootstock, the water-melon yield significantly improved in ‘Xiuli’ (by 26.7 and 87.4%) and ‘Wanzhen No. 2’ (by 16.3 and 89.2%), when compared to both the control ‘Jiashi’ and the non-grafted plant. The yield of ‘Xinong No. 8’ significantly increased (by 32.2%), when compared to the non-grafted plant. Our study suggests that ‘Wanzhen No. 2’ is a suitable rootstock for grafted seedlings in three watermelon cultivars. We have provided new insights into the rootstock and contributed to the improvement of watermelon grafted plants.